1. Field of the Invention
The present invention relates to an optical lens design and more particularly, to an optical lens for lighting fixture.
2. Description of the Related Art
Taiwan Patent No. M397532 discloses an optical lens for streetlight, which mainly uses the outer curved surface of the lens to achieve a rectangular light distribution pattern. Further, Taiwan Patent No. M378351 discloses a convex lens for LED light source, which has the outer curved surface of the lens configured to shift the illumination area and to satisfy the desired distribution of light emission. Further, Taiwan Patent No. M380480 discloses an asymmetric streetlight LED lens, which also has the outer curved surface of the lens configured to shift the illumination area and to satisfy the desired distribution of light emission.
However, the aforesaid prior art techniques do not disclose any inner curved surface lens design to achieve a rectangular light distribution pattern,
As shown in FIGS. 15 and 16, the isolux distribution of a conventional LED streetlight has approximately an oval-shaped pattern. In this oval-shaped pattern, the corner area has a relatively low illumination, i.e., non-uniform illumination.
As shown in FIG. 17, if the conventional LED streetlight has a height of 8 m and an illumination range of 24 m×16 m (as indicated by the imaginary line), the luminous flux will be 5000 lm, and the central illuminance will be 28.0 lux.
From FIGS. 15 and 16, it can be known that the corner illuminance is about 1˜3 lux The lower corner illuminance may affect uniformity of illumination. Further, the light utilization efficiency of this conventional design of LED streetlight is about 60%, and its uniformity of illumination is about 0.4, i.e., this conventional design of LED streetlight has a lower uniformity in illumination and a low light utilization efficiency.